1. Field of the Invention
The present invention generally relates to an optical transmission system that can be applicable to a long distance, high capacity transmission.
The present invention also relates to optical transmission devices, such as a transmitter and a receiver for the optical transmission system.
Such types of high capacity transmission systems using optical signals have been developed and designed so as to be adapted to multimedia applications. Many TDM (Time Division Multiplexing) transmission systems or WDM (Wavelength Division Multiplexing) transmission systems have been known. Typically, those systems have been intended to efficiently make use of a transmission line. In these high capacity transmission systems, it is particularly demanded that a reliable transmission can be achieved.
Therefore, the present invention relates to, in particular, the optical transmission system that has their transmission reliability been improved and the optical transmission device used in this system.
2. Description of the Related Art
A conventional wavelength-multiplexing transmission system includes an optical transmitter 201, an optical transmission line 203 and an optical receiver 202, as schematically shown in FIG. 1, and the system is in conformity with SDH (Synchronous Digital Hierarchy) that is a set of international, digital transmission standards. The optical transmitter 201 has, for each of k channels CHi (i=1, . . . , k), individually an SOH (Section Over Head) inserting unit 204 for inserting an SOH, an electrical-optical converter (OS) 205 and a wavelength-multiplexer 206. The optical receiver 202 also has, for each of the k channels, individually a wavelength-demultiplexer 207, an optical-electrical converter (OR) 208 and an SOH terminating unit 209.
The SOH inserting unit 204 at the optical transmitter 201 inserts the SOH into an electrical signal for one of the corresponding channels CHi. Each electrical signal for the every channel is then provided to the optical-electrical converter 205 and converted to an optical signal with a wavelength λi corresponding to the channel CHi. The optical signals having the wavelength of λi are multiplexed by the wavelength-multiplexer 206 and resulting wavelength-multiplexed signals are transmitted to the optical transmission line 203.
The wavelength demultiplexer 207 at the optical receiver 202 separates the multiplexed signals received from the optical transmitter 201 through the optical transmission line 203 into the signals corresponding to the wavelengths λ1 to λk, respectively. These optical signals having the wavelength of λ1 to λk, respectively, are converted to corresponding electrical signals by the optical-electrical converter 208, and then the SOH of the electrical signals is terminated by the SOH terminating unit 209. The electrical signals having their SOH terminated are transmitted to a further stage (not shown in FIG. 1) on an each i.e., (individual) channel basis. Thus, the data comprising the electrical signals for each of the channels CH1 to CHk can be transmitted from the optical transmitter 201 to the optical receiver 202 over the signal optical transmission line 203.
Several error correction techniques have been also proposed in order to improve a transmission quality by correcting transmission errors involved in the transmitted data. For example, one of the known techniques, also called an “FEC (Forward Error Correction)” method, consists in generating and adding an error correction bit to the data representing one frame or the data of a predetermined length and performing the error correction at a receiver side.
Adding a parity bit to the transmitted data is also a common technique used for determining a presence/absence of the transmission error within the transmitted data. In this case, the SOH may be also provided with error monitoring bits, named B1 and B2.
The earlier described error correction techniques consist in, for every frame or every block of the transmission data, generating an error correction bit and adding it to each frame or block. Therefore, in contrast with a transmission system without correcting transmission errors, the conventional transmission system provided with the error correction technique has to increase a transmission rate, because a number of bits to be transmitted are increased. Alternatively, if the transmission rate is set to a predetermined value, the transmission system should reduce an amount of the transmission data so that the error correction bit can be transmitted together with the transmission data within the predetermined transmission rata.
Furthermore, in some of the conventional transmission systems, erroneous bits included in the transmission data cannot be corrected when parity bits are contained in the data. One solution for improving a capability of correcting the erroneous bits in the data is to increase the number of the error correction bits added to the transmission data. However, this solution may be not practical, because a considerably high transmission rate is required for increasing the number of error correcting redundant bits to be added to the transmission data.
Another possible solution is to insert the error correction bits into reserved bits within the SOH. The reserved bits means that those bits are reserved for a variety of future applications. In this case, since a lot of redundant bits are to be inserted into some particular locations in the SOH, a problem may occur that a size of a circuit comprising a transmission device, such as the transmitter 201 and the receiver 202, is enlarged. This solution has a further drawback in that the error correction bits, which have been already assigned to the reserved bits, cannot be made use of, if the reserved bits are decided to be used for one of the future appiications.